1. Field
This invention relates in general to spent nuclear fuel pools and, more particularly, to an emergency cooling system to maintain the safety of spent nuclear fuel pools.
2. Related Art
Pressurized water nuclear reactors are typically refueled on an eighteen month cycle. During the refueling process, a portion of the irradiated fuel assemblies within the core are removed and replaced with fresh fuel assemblies which are relocated around the core. The removed spent fuel assemblies are typically transferred under water to a separate building that houses a spent fuel pool in which these radioactive fuel assemblies are stored. The water in the spent fuel pools is deep enough to shield radiation to an acceptable level and through convective cooling and recirculation prevents the fuel rods within the fuel assemblies from reaching temperatures that could breach the cladding of the fuel rods, which hermetically house the radioactive fuel material and fission products. Cooling continues at least until the decay heat within the fuel assemblies is brought down to a level where the temperature and radiation emissions of the assemblies is acceptable for dry storage. Until such time, the water in the spent fuel pools is actively cooled by pumping a portion of that coolant through heat exchangers to extract the heat.
Current nuclear plants have acquired systems to effectively manage spent fuel cooling. However, certain emergency situations, such as a station blackout or an event causing the loss of an ultimate heat sink can lead to failure of the spent fuel pool cooling process. As a consequence, extensive temperature increase of the spent fuel pool inventory with the formation of steam can occur, and radioactive aerosols can possibly be released into the surrounding atmosphere. High concentrations of this radioactive aerosol and overheating of the air may lead to limited accessibility to the building and further impede emergency efforts.
In an extremely unlikely event, such as where a catastrophic tsunami challenges plant systems beyond all reasonable predictions, such as occurred in Japan's Fukushima Daiichi Nuclear Power Plant, and where there is no cooling or inventory make-up for several days, the spent fuel pool may vaporize significant volumes of water and eventually dry up, posing the risk of fuel damage due to the inability to reject decay heat to a heat sink.
Currently, a number of existing plants have a spray system incorporated on top of the spent fuel pool, which distributes water from an external water source to replenish water in the pool. However, the system requires a pump which requires power and manual start-up to carry out this operation.
Accordingly, it is an object of this invention to provide a back-up spent fuel pool cooling system that does not require external power.
Furthermore, it is an object of this invention to provide such a cooling system that will initiate cooling of the spent fuel pool passively upon the occurrence of a preselected event.
Additionally, it is an object of this invention to provide such a cooling system that is relatively inexpensive and easy to implement.
It is a further object of this invention to provide such a cooling system that is not going to hinder normal fuel pool operations, when the system is not in use.